Steps to reproduce:
Good news: If you move the entire Add content menu item with its children from navigation to, another menu, the node/add page still works...
Bad news: ...the node/add page still works but includes options only for any "Add {content_type}" sub-menu that its respective menu item was moved along with the parent Add content menu item. Any custom content types created after the move of the menu keep adding their respective "Add {content_type}" menu items under the Navigation menu. So, the effect caused is that people wonder why their custom content types don't get a "Add {content_type}" entry in the "Add content" page and a submenu under the "Add content" menu item.
Also, you can disable the Add content link in Navigation, its children still show up there and on node/add.
Bad news: If you disable a node/add/[content-type] menu link, it disappears from the node/add page too.
This is a regression from Drupal 6, as i remember disabling less important content types from the menu and still being able to access them on the node/add page.
Fixed
8.0 ⚰️
Not all content is available!
It's likely this issue predates Contrib.social: some issue and comment data are missing.
There is now a best solution in contrib (Drupal 7 and modern Drupal) that makes it easy again to modify the links on the node/add page if you want to, providing a separate menu to power a replacement "Add content" page:
https://www.drupal.org/project/custom_add_content →
(Glad i found that module as i was about to make the equivalent, since it was my fault it got difficult by reporting this bug!)